Foams based on polyisocyanates such as polyurethane foams having an impervious outer skin and a cellular core, of the type obtained by the in-mold foaming technique (note, e.g. German Auslegeschrift No. 1,196,864 and French Pat. No. 1,559,325), are eminently suitable for the mass production of lightweight structures, such as those used in furniture making, vehicle construction and house building.
Such polyurethane moldings are generally produced by introducing the foamable reaction mixture consisting of polyisocyanates, compounds with at least two isocyanate-reactive hydrogen atoms, and additives, into closed temperable molds in which it foams and solidifies under heavy compression. The foamable reaction mixture exactly fills the mold and reproduces the inner surfaces thereof.
It is generally preferred to use molds made from a material with as high a heat capacity and as high a thermal conductivity as possible, preferably metal. However, it is also possible to use other materials, such as plastics, glass, wood, and the like.
In order to prevent the molding from adhering to the surface of the mold when the mold is opened, the mold is coated with a release agent. Examples of release agents currently in use include waxes, soaps or oils. These external release agents form a thin film between the surface of the mold and the molding, which film does not adhere either to the mold or to the plastic material, and thus enables the molding to be readily removed from the mold.
This method has a number of disadvantages where mass production is desired. The release agent has to be applied at regular intervals, and, during that period, the mold is out of service. Fine engravings on the mold surface such as imitation wood or leather grain, become covered with residues of release agent over a period of time. The removal of these firmly adhering residues from the, often, very complicated molds involves considerable time and expense. The plastic moldings are coated with a thin film of release agent to which lacquer systems are unable to adhere. Before lacquering, the moldings have to be ground or treated with solvents in order to obtain adequate adhesion between the lacquer and the plastics material.
It is known that the need to coat the mold with a release agent may be eliminated by adding to the foamable reaction mixture, certain additives which provide the finished molding with outstanding release properties in metal molds, (See, e.g., U.S. Pat. No. 3,726,952). Additives of this type include salts, containing at least 25 carbon atoms, of aliphatic carboxylic acids with primary amines or amines containing amide or ester groups.
German Offenlegungsschrift No. 2,121,670 relates to a process for the production of foams by foaming a reaction mixture of polyisocyanates, compounds containing reactive hydrogen atoms, water and/or organic blowing agents and additives in a closed mold, wherein a mixture of (a) salts, containing at least 20 aliphatic carbon atoms, of aliphatic carboxylic acids and, optionally, amines containing amide and/or ester groups, and (b) natural and/or synthetic oils, fats or waxes is added to the foamable mixture.
Since agents of this type internally lubricate the plastics mixture, they also provide the plastics material with outstanding flow properties in the mold with reduced bubble formation on the plastics surface. In addition, these internal release agents have an antistatic effect and show outstanding release properties even in metal molds with heavily textured surfaces.
Although outstanding release effects in the case of hard plastics may be obtained by conventional processes, it has frequently been found in practical application that the esters of higher fatty acids or the mixed esters thereof, often used as synthetic oils or waxes, still produce inadequate release properties in cases where certain elastomeric foam plastics are to be obtained. The same also applies to the use of the aforementioned salts of aliphatic carboxylic acids.
Accordingly, there was a need to find internal release agents of the type which also develop outstanding release properties in the production of elastomeric polyurethane foams with a cellular core and impervious outer skin.
In the search for release agents with a more favorable release effect with elastomeric, polyurethane foams produced by the in-mold foaming technique, it has been found that additions of higher carboxylic acids, such as oleic acid or other unsaturated higher fatty acids, especially natural or synthetic, commercial-grade fatty acid mixtures containing oleic acid, produce a certain release effect when added to the polyol side of the reaction mixture. Unfortunately, this effect is extremely difficult to exploit in practice because the addition of acid in the quantities required for effective separation seriously retards the reaction of the system, especially the reaction in the initial stages of foam formation. Additionally, both cell structure and stability of the foams are undesirably affected. Finally, the mold-release times of the molding increase and the initial stiffness of the molding is too low.